Various environmental and industrial chemicals can disturb male reproductive function. The objective of these studies is to enhance our understanding of that toxic potential, and to further elucidate the mechanism of action in chemicals found to be toxic. Chemicals which are active as chemosterilants in males, such as dibromochloropropane and the phthalate esters, are used in various test systems. In addition to mechanistic studies, chemicals of unknown activity, such as the phenoxy herbicides and TCDD, have also been studied. Endpoints of toxicity include the assessment of spermatogenesis, sperm morphology, and hormone levels. Studies are beginning on the morphological response of the testis to chemical exposure. Androgen Binding Protein (ABP) assays will also be performed to assess Sertoli cell function. These studies are expected to yield valuable data on chemical toxicity in males, as well as improve the sensitivity and accuracy of future testing systems.